1. Technical Field
The invention relates generally to roller cone drill bits. Particularly, the invention provides new flat profile cutting element geometries for roller cone bits.
2. Background Art
Roller cone drill bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a roller cone drill bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a wellbore 14. Connected to the end of the drill string 12 is a roller cone-type drill bit 20.
As shown in FIG. 2, roller cone bits 20 typically comprise a bit body 22 having an externally threaded connection at one end 24, and a plurality of roller cones 26 (usually three as shown) attached at the other end of the bit body 22 and able to rotate with respect to the bit body 22. Disposed on each of the cones 26 of the bit 20 are a plurality of cutting elements 28 typically arranged in rows about the surface of the cones 26. The cutting elements 28 can be tungsten carbide inserts, polycrystalline diamond inserts, boron nitride inserts, or milled steel teeth. If the cutting elements 28 are milled steel teeth, the teeth may be coated with a hardfacing material.
Prior art roller cone bits generally have cutting elements arranged so that they contact a formation in an arcuate cross section or xe2x80x9cprofile.xe2x80x9d An example of such a prior art bit is shown in FIG. 3A. FIG. 3A shows a cross section through roller cones 30 of a drill bit 31. The cross sectional view shows a cutting element profile 36 generated when cross sections of all the cones 30 of the bit 31 are rotated into the same plane. In FIG. 3A, the roller cones 30 are rotatably attached to legs 32 of the drill bit 31. The cutting elements 34 are arranged about the surface of the roller cones 30. The cutting elements 34 in contact with the bottom of the drilled hole are further illustrated in FIG. 3B.
FIGS. 3A and 3B show that prior art bits generally have cutting elements 40-43 arranged in an arcuate cutting element profile 36 so that the bit 31 drills a wellbore with a similarly arcuate, rounded bottom hole profile (38 in FIG. 3B). The cutting element profile 36 is defined as a curve or line that connects crests 33 of the cutting elements 40-43 and that defines the relative shape of the bottom of the hole drilled by the cones 30. The cutting element profile 36 may be further defined by angular measurements taken at points (such as points A, B, and C in FIG. 3A) along the profile 36. Points A, B, and C are located at midpoints of crests 33, and angular measurements are defined relative to a horizontal plane (not shown). In FIG. 3A, point A is located at the midpoint of a gage cutting element and point C is located at the midpoint of a centerline cutting element. The angular measurements with respect to the horizontal plane at points A, B, and C are 17.5 degrees, 0 degrees, and 14.0 degrees, respectively.
FIG. 3B shows a planar cross sectional view similar to FIG. 3A. FIG. 3B also shows that the cutting elements of prior art bits typically have crests 44 and 45 that are disposed at various angles with respect to a bit axis of rotation 46 when the cutting elements 40-43 are drilling the formation 39. Therefore, when prior art bits contact the formation with arcuate profiles and at the angles defined by the crests of the cutting elements, the contact between the cutting elements and the formation is generally non-uniform.
The invention is a drill bit that includes a roller cone and a plurality of cutting elements. The roller cone is affixed to a bit body-and is arranged circumferentially about an axis of rotation of the bit.
One aspect of the invention includes cutting elements that are arranged so that the crests of at least half the cutting elements are substantially perpendicular to the bit axis of rotation when the cutting elements are in a downwardmost rotary orientation. In one embodiment, the crests are within about 10 degrees of perpendicular to the axis of rotation. In another embodiment, the crests are within about 5 degrees of perpendicular to the axis. In a particular embodiment, substantially all the crests are within about 10 degrees of perpendicularity to the bit axis.
In another aspect of the invention, the roller cone and the cutting elements are arranged so that the crests on the cutting elements define a substantially flat profile. Substantially flat includes profile angles, with respect to perpendicular to the bit axis, at either the gage edge of the bit or at the bit centerline of 11 degrees or less.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.